Device for Measuring the Movement of a Self-Guided Vehicle

ABSTRACT

A device for measuring the movement of a self-guiding vehicle, that has an enhanced measuring reliability, in particular during an adhesion loss and independently from the travel profile of the vehicle in terms of slope, turn and slant. To this end, the device for measuring the movement of a self-guiding vehicle includes on board thereof two accelerometers coupled to a movement calculator, wherein each accelerometer includes two measurement axes on which are measured projections of a vehicle acceleration resultant. The four measurement axes of the accelerometers are adjusted so that the calculator provides, from the four projection measures, at least one very accurate longitudinal acceleration value of the vehicle at each point of a route including both slopes and turns.

The present invention relates to a device for measuring the movement ofa self-guided vehicle according to the preamble of claim 1.

Numerous methods or devices for measuring movement, speed oracceleration of a vehicle are known nowadays, in particular for vehiclesintended for public transport such as a carriage of a train, of a metrotrain, a trolleybus, a tramway car, a bus or any other vehicle driven intraction by at least one trackway or a rail such as a guide rail. Inparticular, in the case of a vehicle which is self-guided by a trafficsystem (railway signals, onboard and/or remote autopilot system of thevehicle, etc.), measures to provide self-guidance which is reliable(against breakdown) and safe (for the passengers or goods) areindispensable whatever the nature of the route of the vehicle. In thissense, it is essential to be accurately informed in real time about theposition, the speed (and the acceleration) of the vehicle, in particularin situations where the vehicle incurs unavoidable loss of adhesion,such as during slipping (during acceleration of the vehicle) or wheellocking (during braking of the vehicle) of the free-running measurementaxle or drive axle.

When the guided vehicle has an axle which is free of any tractive orbraking force, the movement of the vehicle is directly provided by therotation of the axle (or one of the wheels associated with this axle).

However, this solution reduces the tractive or braking power and thusthe performance of the vehicle, and this is why the majority of systemsdo not have free-running axles.

In the absence of a free-running axle and to overcome the consequencesassociated with slipping/wheel locking, with loss of adhesion of one ofits wheels, several devices exist and use:

-   -   either measuring means which are completely independent of the        wheels permitting a measurement of speed by optical means or        even by means of a Doppler effect radar system. These costly        devices generally use, however, an additional tachometer for        operation at low speed and when the vehicle is stationary, said        tachometer making it possible to obtain the angular speed of a        wheel or the number of revolutions of the wheel per unit of        time;    -   or inertial units combining accelerometers, gryometers and        terrestrial localization systems such as a GPS. Said systems,        however, remain very costly due to their high-level technology,        frequently used in applications for aeronautical systems;    -   or, such as in EP 0 716 001 B1, a single tachometer arranged on        an axle and a means for taking into account a safety margin for        the values measured on one wheel or on the wheels in order to        attempt to compensate for the effects of possible slipping/wheel        locking which impairs the performance for measuring movement as        it still remains too approximate. This also results in an        anti-lock system as compensation which may be very abrupt for a        vehicle and its passengers or goods;    -   or, such as in US 2005/0137761 A1, an accelerometer fitted in        the vehicle and a tachometer on an axle, the measurement signals        of which are linked to an appropriate central computer, even if        not specifically disclosed, to take into account errors which        occur in the event of loss of adhesion and to provide the speed        and the position of the vehicle on its route. In particular, the        accelerometer comprises two measurement axes in order        respectively to determine an acceleration in one direction of        the trajectory of the vehicle and in order to determine and thus        take into account, in the calculation of movement, a slope of        the vehicle relative to a horizontal plane. Values of the        measurement signals of the accelerometer and of the tachometer        are also compared to threshold speed values which, if they        exceed a threshold, make it possible to indicate the presence of        loss of adhesion (slipping/wheel locking) of the vehicle.        Although the effects of slope sustained by the vehicle are taken        into account, other effects associated with the trajectory of        the vehicle are inevitable depending on the position of the        accelerometer (and of the positioning of the two measurement        axes thereof) in the vehicle, as a railway transport unit        frequently has an elongate geometry along which a single        accelerometer and a tachometer placed upstream of the vehicle        may not provide a measurement means which shows the effects        acting on the complete assembly of the vehicle such as, for        example, the effects of turn or lateral acceleration.

All these devices make it possible, therefore, to calculate the movementof a guided vehicle, which does not have axles which are free of anybraking and tractive force, and which runs on a track of any profile butwith an accuracy which is much lower than that of an “ideal” system witha free-running axle, as they may not completely overcome losses ofadhesion (slipping and wheel locking caused by tractive/braking forces)in addition to errors caused by lateral acceleration (turn, slant) andeven vertical acceleration (slope).

One object of the present invention is to propose a device for measuringthe movement of a self-guided vehicle that has enhanced measuringreliability, in particular during an adhesion loss and independentlyfrom the travel profile of the vehicle in terms of slope, turn andslant.

To this end, a device for measuring the movement of a self-guidedvehicle comprising two on-board accelerometers, each including twomeasurement axes and of which the measurement signals are coupled to acomputer for calculating the movement, is proposed as claimed in claim1.

As one possibility, at least one tachometer may be mounted on one of theaxles of the vehicle and also coupled to the computer for processingdata thus provided from all the sensors (accelerometers and tachometer).The measurement signals delivered by the tachometer may be utilized toimprove the accuracy of the device.

The device according to the invention, based on accelerations measuredon the measurement axes, provides data regarding the speed andlongitudinal movement of the vehicle (for example along a railwaytrack). It may be associated with any type of on-board device likely torequire an accurate and continuous measurement of the speed and of themovement of the vehicle, irrespective of the conditions of rail/wheeladhesion and whatever the profile of the route in terms of slope, turnand slant.

The accelerometers and their measurement axes are arranged such that,based on measurements taken on the different measurement axes, theypermit longitudinal acceleration, lateral acceleration and slopeacceleration of the vehicle to be calculated in order to determinesubsequently the speed and the longitudinal movement of the vehicle bythe integration of time onto the acceleration values.

The device according to the invention also advantageously makes itpossible to detect in a reliable manner an immobilization of the vehicleon its route and produces to this end information about zero speed frominformation delivered by the sensors.

The device comprises a means for auto-calibration and auto-testing whichmakes it possible, when the vehicle is immobile, to verify the correctfunctioning of the sensors and as a result to guarantee with a highdegree of reliability data made available by other on-board systems.

One appropriate use of the device according to the invention covers thefield of guided vehicles whatever their type of guidance (mechanical orintangible, i.e. without a mechanical connection between the ground andthe vehicle) in particular trains, metro trains, tramway cars or busesand whatever the type of operation (axles, bogies) with iron wheels ortires. It is noteworthy here that for this category of vehicle with anelongate geometry/chassis, the effects of turn and slope are notnegligible, depending on the position (or the offset) of theaccelerometers on-board the vehicle. The invention thus advantageouslypermits these effects to be overcome in order to determine the movementof the vehicle more accurately.

The device according to the invention thus makes it possible tocalculate the movement of a guided vehicle which does not have axlesfree of any braking and tractive force, and which runs on a track of anytype of profile, maintaining an accuracy which is equivalent to that ofa system with free-running axles, whilst overcoming loss of adhesion(slipping and wheel locking caused by tractive/braking forces) anderrors caused by lateral acceleration (turn) and vertical acceleration(slope).

A set of sub-claims also presents advantages of the invention.

Exemplary embodiments and examples of application are provided withreference to the figures described, in which:

FIG. 1 shows a vehicle provided with a device for measuring movement ofthe self-guided vehicle according to the invention,

FIG. 2 shows a diagram for defining the planes associated with thevehicle in motion,

FIG. 3 shows a diagram for taking into account the effect of slope onthe device,

FIG. 4 shows a diagram for taking into account the effect of turn on thedevice.

FIG. 1 shows a vehicle VEH provided with a device for measuring themovement of the self-guided vehicle according to the invention and,possibly associated with FIG. 2, clarifying how the planes associatedwith the vehicle in motion are defined according to the accelerationsustained by the vehicle and measured by two accelerometers 101, 102.FIGS. 3 and 4 show the arrangement of measurement axes Acc1, Acc2, Acc3,Acc4 of the accelerometers according to the planes selected according tothe type of acceleration Gx, Glat, Gpes (longitudinal movement, effectof turn or/and of slope) sustained by the vehicle as a co-ordinate (X,Y, Z) centered on the accelerometers and of which the axis X indicatesthe direction of the longitudinal trajectory of the vehicle.

The device for measuring movement (real-time position Dx) of theself-guided vehicle VEH comprises on-board thereof:

-   -   an accelerometer 101 provided with two measurement axes Acc1,        Acc2, in a longitudinal plane Py defined by a first longitudinal        axis X according to a principal movement VEx, assumed to be        rectilinear, of the vehicle and a second axis Z perpendicular to        the floor of the vehicle,    -   a computer 103 connected to an output signal S1, S2 associated        with each measurement axis Acc1, Acc2 where each output signal        S1, S2 includes a measurement as an orthogonal projection Gacc1,        Gacc2 of a total acceleration resultant of the vehicle on the        associated measurement axis Acc1, Acc2,    -   a second accelerometer 102 being provided with at least two        measurement axes Acc3, Acc4 in a horizontal plane Pz defined by        the first axis X and a third axis Y perpendicular to the first        and to the second axis X, Z,    -   the computer 103 is connected to an output signal S3, S4        associated with each measurement axis Acc3, Acc4, where each        output signal S3, S4 includes a projection measurement Gacc3,        Gacc4 of the total acceleration resultant of the vehicle on the        associated measurement axis Acc3, Acc4,    -   all the measurement axes Acc1, Acc2; Acc3, Acc4 of the first and        of the second accelerometer 101, 102 have in their respective        plane Py, Pz an adjustable relative angle A1+A2, A3+A4 which is        thus adjusted so that the computer 103 provides from the four        projection measurements Gacc1, Gacc2, Gacc3, Gacc4, at least one        instantaneous value of longitudinal acceleration Gx of the        vehicle at each point of a route including both slopes and        turns. In other words, the value of longitudinal acceleration Gx        is an exact acceleration value, taking into account the effects        of slope and turn. Similarly, a loss of adhesion leading to the        falsification of an acceleration measurement which would be        deduced from the rotation of the axles, may be ideally        compensated here.

Chiefly, therefore, the device according to the invention uses twobi-axial accelerometers 101, 102 fixed to the body of the vehicle andintended to measure a longitudinal acceleration and a lateralacceleration of the vehicle. The vehicle is subjected to three forcesproducing a longitudinal acceleration Gx (movement of the vehiclesubjected to tractive/braking forces), a lateral acceleration Glat (theturn of the trajectory causing centrifugal acceleration) and a verticalacceleration Gpes due to the gravity which is exerted in the presence ofa slope (the slope of the trajectory). The first accelerometer 101 ofwhich the two axes Acc1, Acc2 are located in the vertical plane Py andthe second accelerometer 102 of which the two axes Acc3, Acc4 arelocated in the horizontal plane Pz, make it possible to measure aresultant of the accelerations (longitudinal, lateral, gravity)projected on each of the four measurement axes. The angles between thedifferent measurement axes of the accelerometers are known and fixedafter adjustment. The computer 103 solves a system composed of fourequations in order to determine four unknowns at the position Dx of thevehicle, namely a slope angle Ax of the trajectory, a lateralacceleration angle Ay (resultant of the centripetal force due to thespeed of the vehicle and dependent on the radius of curvature R of thetrajectory in addition to the offset of the accelerometer relative tothe center of the vehicle), a value of lateral acceleration Glat and thevalue of longitudinal acceleration Gx. By successive integration overthe duration of the journey, the computer 103 determines thelongitudinal speed Vx and the longitudinal movement Dx of the vehicleVEH over its route for any slope and turn COURB.

If required, the device according to the invention is complemented by atachometer 108 to improve the accuracy of the above measurement of thespeed Vx and of the distance Dx covered. The tachometer 108 is fixed toone of the axles R1 a, R2 a, R1 b, R2 b of the vehicle VEH and itsoutput signal(s) STb (is)are transmitted to the computer 103. Thecomputer 103 evaluates a movement DxT and a speed VxT based onmeasurement signal(s) of the tachometer. The computer carries out acomparison between the results of the measurement of movement from thetachometer and those from the accelerometers. When for these measuredvalues, a difference in measurement is lower than a threshold, themeasurement values are reset to those of the tachometer. In the oppositecase (value greater than a threshold) there is no correction of theresults from the measurement of the accelerometers.

As shown in FIG. 1, zero speed information Op may also be reliablyprovided by the computer 103 from information Im originating fromequipment of the vehicle (immobilization signal, zero speed indicator,etc.) or be determined by the device according to the invention itself.To determine this information, the computer 103 processes theinformation from the tachometer and the accelerometers.

When the device determines zero speed and due to the specifics of theproposed mounting of the accelerometers, the device also advantageouslyhas the capacity to implement an auto-test function. This auto-testfunction makes it possible to evaluate the corrections which have to bemade to the measurements from the accelerometers (afterauto-calibration) and to identify faults in the operation of theaccelerometers. The multiplicity of the measurement axes provides aredundancy which is very advantageous for several measurements (due tothe two bi-axial accelerometers) and makes it possible by a periodicverification of reliability of the accelerometers (for example at eachstop at a station) to guarantee test measurements (and thus subsequentmovement) with a very low probability of error, making them compatiblewith the safety demands of a reliable system as required in the railwayfield.

In the remainder of this description, reference is made to the two FIGS.3 and 4.

Considering the measurement axes Acc1, Acc2 of the first accelerometer101 (see FIG. 3 where, for reasons of clarity, the lateral accelerationGlat has deliberately been omitted), the components of the projectionmeasurements Gacc1, Gacc2, by adding the projections of theaccelerations Gx, Glat, Gpes on each of the axes Acc1, Acc2 of theaccelerometer 101 are:

-   -   on the axis Acc1

Gacc1=projection (Gx)−projection (Gpes)−projection (Glat)

Gacc1=Gx cos(Ay)cos(A1)+Gpes sin(A1−Ax)−Glat sin(Ay)cos(A1)  (1)

-   -   on the axis Acc2

Gacc2=projection (Gx)−projection (Gpes)−projection (Glat)

Gacc2=Gx cos(Ay)cos(A2)−Gpes sin(A2+Ax)−Glat sin(Ay)cos(A2)  (2)

Similarly, considering the measurement axes Acc3, Acc4 of the secondaccelerometer 102 (see FIG. 4 where, for reasons of clarity, the slopeacceleration Gpes has deliberately been omitted), the components ofprojection measurements Gacc3, Gacc4 by the addition of the projectionsof the accelerations Gx, Glat, Gpes on each of the axes Acc3, Acc4 ofthe accelerometer 102 are:

-   -   on the axis Acc3

Gacc3=projection (Gx)−projection (Glat)−projection (Gpes)

Gacc3=Gx cos(A3+Ay)−Glat sin(A3+Ay)−Gpes sin(Ax)cos(A3)  (3)

-   -   on the axis Acc4

Gacc4=projection (Gx)−projection (Glat)−projection (Gpes)

Gacc4=Gx cos(A4−Ay)+Glat sin(A4−Ay)−Gpes sin(Ax)cos(A4)  (4)

With for the equations (1) to (4):

-   -   the angle A1 in the plane Py between the axis X and the axis        Acc1    -   the angle A2 in the plane Py between the axis X and the axis        Acc2    -   the angle A3 in the plane Pz between the axis X and the axis        Acc3    -   the angle A4 in the plane Pz between the axis X and the axis        Acc4    -   the angle Ax of the trajectory of the vehicle in the plane Py        (i.e. the angle between the horizontal and the axis X)    -   the offset distance Dx between the center of the vehicle and the        fixing point of the accelerometers 101, 102 installed on the        vehicle    -   the angle Ay associated with the radius of curvature R in the        plane Py. The angle Ay is calculated by Arctg (Lx/R), thus in a        first approximation Lx/R, given that the value of the radius of        curvature R is usually greater than the offset distance Lx.

The resolution of the system formed by the four equations (1) to (4)falls within the scope of mathematical techniques which are notdisclosed here and of which the object is to calculate the fourvariables Gx, Glat, Ax and Ay according to the measurements ofacceleration values Gacc1, Gacc2, Gacc3, Gacc4 of which the computer 103makes use.

However, the resolution of the system is advantageously simplified incertain specific hypotheses for the arrangement of the accelerometers101, 102.

From these hypotheses may be selected the relative angles A1+A2, A3+A4each defining a right angle, i.e.: A1+A2=90° and A3+A4=90°. Thus, thedevice according to the invention may provide that at least one of therelative angles A1+A2, A3+A4 is a right angle.

The device according to the invention is implemented such that eachrelative angle A1+A2, A3+A4 is in fact subdivided (or subdivisible) intoa first and a second angle A1, A2 and respectively A3, A4 correspondingto projection angles between the four measurement axes Acc1, Acc2, Acc3,Acc4 of the first and of the second accelerometer 101, 102 and the firstaxis X (longitudinal axis according to a principal movement of thevehicle, assumed to be rectilinear).

In this regard, it is also very advantageous to select the angles A1,A2, A3, A4 so that A1=A2 and A3=A4, and in particular so thatA1=A2=A3=A4=45°.

Regarding the choice of angles A1, A3, it is also possible to attributeto them adjustable values making it possible to estimate in the bestpossible manner the effects of slope or turn without impairing theaccuracy of the measurement of longitudinal acceleration.

By way of example, if the option is selected in which the projectionangles A1, A2; A3, A4 of each accelerometer are equal, i.e. Al=A2 andA3=A4, the above system of equations becomes:

Gacc1=Gx cos(Ay)cos(A1)+Gpes sin(A1−Ax)=Glat sin(Ay)cos(A1)  (1)

Gacc2=Gx cos(Ay)cos(A1)−Gpes sin(A1+Ax)−Glat sin(Ay)cos(A1)  (2)

Gacc3=Gx cos(A3+Ay)−Glat sin(A3+Ay)−Gpes sin(Ax)cos(A3)  (3)

Gacc4=Gx cos(A3−Ay)+Glat sin(A3−Ay)−Gpes sin(Ax)cos(A3)  (4)

The resolution of this system makes it possible to determine easily thefour unknowns which are sought and defined by the variables Gx, Glat,Ax, Ay, then by integration over a duration of movement to deducetherefrom the longitudinal speed Vx and the associated position Dx overthe route of the vehicle:

-   -   Vx=∫ (Gx dt)    -   DX=∫ (Vx dt)

The device according to the invention thus permits the computer 103 toprovide a value of the slope angle Ax, of a lateral acceleration angleAy (i.e. representing the rotation of the lateral acceleration at thefixing point of the mounting of the accelerometer relative to which itwould be at the center of the vehicle for the radius of curvature R) ateach point of the route which includes both slopes and turns.

By extension, the computer 103 provides a speed Vx and a position Dx ateach point of the route which includes both slopes and turns byintegrating successively the value of longitudinal acceleration Gx ofthe vehicle.

As disclosed above, the device may also comprise:

-   -   a tachometer 104 arranged on at least one axle of the vehicle        and providing a tachymetric value of speed VxT and position DxT        of the vehicle,    -   the tachymetric values VxT, DxT and the speed and position        values Vx, Dx obtained and respectively delivered by the        computer 103 are provided to a comparator 106 incorporated in        the computer 103,    -   the comparator 106 determines the differences between the        categories of speed and position values, and if said differences        are below a predefined threshold, a resetting of the speed and        position values Vx, Dx provided by the computer 103 at each        point of the route which includes both slopes and turns is        implemented on the tachymetric values VxT, DxT. If the        difference is above the threshold, the resetting is inhibited.

This possibility of resetting provides an increase in the accuracy ofmeasuring the speed and movement based on a simple additionalmeasurement of speed and movement which is proportional to the radius ofthe wheel.

The device according to the invention may also comprise a means fordetecting zero speed 107 of the vehicle which is incorporated in orcoupled to the computer 103 and to the tachometer 104. Said tachometercomprises at least one correlator of the speed and position values Vx,Dx delivered by the computer 103 and corresponding tachymetric valuesVxT, DxT.

As a result, a very reliable function for detecting zero speed isimplemented, namely:

-   -   by taking into account information which is external to the        device made available by one of the devices of the vehicle (for        example by means of an internal signal of the immobilized        vehicle, etc.)    -   by determining a stoppage of the vehicle by filtering        information about speed and movement Vx, Dx provided by the        computer 103. This determination may thus be correlated with the        corresponding tachymetric data VxT, DxT.    -   following this processing, if it is certain that the vehicle is        genuinely stopped, the device provides so-called zero speed        information.

A function known as auto-test may thus advantageously use the so-calledzero speed information. When this information is legitimately provided,it means that the vehicle is immobile and as a result, the longitudinaland lateral acceleration are thus zero.

The associated test thus consists in checking that the measurementvalues delivered by the accelerometers 101, 102 verify the system ofequations (1), (2), (3), (4) provided above, which is thus reduced to:

Gacc1=Gpes sin(A1−Ax)  (1)

Gacc2=−Gpes sin(A2+Ax)  (2)

Gacc3=−Gpes sin(Ax)cos(A3)  (3)

Gacc4=−Gpes sin(Ax)cos(A4)  (4)

An example of resolution of this system is provided here in theparticular hypothesis of the arrangement of the accelerometers, forwhich the projection angles A1, A2; A3, A4 are equal for each pair ineach of the planes Py, Pz, i.e. that A1=A2 and A3=A4:

From the two last equations (3) and (4) the following relations (5) and(6) may be deduced:

Gacc3=Gacc4  (5)

Sin(Ax)=−Gacc3/(Gpes Cos(A3))  (6)

Relative to the term Sin(Ax) in the equations (1) and (2), it is thuspossible to verify the measured values of the projected accelerationsGacc1, Gacc2 of the first accelerometer 101 with the above calculatedresults.

The projected accelerations Gacc3, Gacc4 of the second accelerometer 102are verified by the equation (5). In a first approximation, it isreasonable to consider that the slope has little influence on themeasurement which is generally the case, for example, when parking inthe garage or when stopped at the station.

In order to refine the verification of the projected accelerationsGacc3, Gacc4 of the second accelerometer 102 it is, however, alsopossible to read a value of the slope from a data bank.

By these verifications and by selecting a filtering threshold, it ispossible to determine correction factors to be made to the measurementsfrom the accelerometers. In the case of the second accelerometer 102 itis advantageously possible to benefit from the slow process of theaccelerometer drift before modifying its correction factors. Thesecorrection factors are applied following a confirmation obtained afterseveral stops. This number of stops is adjustable according to thedegree of accuracy maintained. This makes it possible to auto-calibratethe device according to the invention.

A second selected threshold which is higher than the first threshold mayalso be defined in order to declare that the device according to theinvention is not in operation.

In order to implement the auto-test function, the device according tothe invention comprises:

-   -   a means for auto-calibration 105 of the accelerometers 101, 102        which may be activated if the means for detecting zero speed        confirms a stoppage of the vehicle,    -   the means for auto-calibration processing the measurements from        the accelerometers 101, 102 and provided by a unit for        calculating accelerations 104 (itself receiving the measures        from the accelerometers 101, 102 and being included in the        computer 103),    -   the means for auto-calibration calibrates the measurements        corresponding to the zero values of the longitudinal        acceleration Gx and lateral acceleration Glat of the vehicle.

The means for auto-calibration 105 has a first control mode forverifying the equality of the measurement values Gacc3, Gacc4 on thesecond accelerometer 102 and a means for recalculating the slope angleAx from which the measurement values Gacc1, Gacc2 of the firstaccelerometer 101 are verified by means of a second control mode. Thus,the verification becomes very reliable and even more so if the slopeangle may be evaluated and confirmed redundantly by known informationwhich is external to the device.

For this embodiment and relative to the auto-test function disclosedabove, beyond a first error threshold arising from results of theauto-calibration means 105, correction factors from the auto-calibrationmeans 105 are thus retransmitted to the calculating unit 104 (moreusually to the computer 103 for calculating the movement).

Similarly, beyond a second error threshold which is less safe than thefirst threshold arising from results of the auto-calibration means 105,an indicator of failure of the on-board measurement is activated.

A simplified model of evaluating the probability of failure of thefunction known as auto-test may thus be implemented considering that,with the stoppage of the vehicle, measurements carried out on themeasurement axes acc1, acc2, acc3, acc4 of the accelerometers 101, 102are obtained redundantly.

Assuming a time interval T between two stops of the vehicle: theprobability of failure Pr of the auto-test function applied to the twomeasurement axes Acc1, Acc21 in the plane Py is defined by:

Pr=λacc1*Aacc2*T

Where the respective failure rates Aacc1 and Aacc2 of the measurementaxes Acc1 and Acc2 of the bi-axial accelerometers are each assumed to beequal to a usually permitted value of 10⁻⁵ in the following calculatedexample:

Where T=60 seconds, Pr=10⁻¹⁰*0.017=1.7*10⁻¹²

Where T=10 minutes, Pr=10⁻¹⁰*0.17=17*10⁻¹²

It is thus apparent that if the vehicle stops periodically andfrequently, the device makes it possible to guarantee a level ofconfidence in the measured data which is required for the safetydemanded in the railway field.

According to this evaluation of the probability of failure of theso-called auto-test function, the device according to the invention maythus comprise a means for evaluating the probability of failure whichmay be activated between two stops of the vehicle and using a redundancymeasurement on the measurement axes of the accelerometers. This means ofevaluation may be integrated in the auto-calibration means 105 disclosedabove.

Finally, the device according to the invention may also optionallycomprise a detector of loss of adhesion of the vehicle (in the case ofslipping or wheel locking) coupled to at least one of the first andsecond bi-axial accelerometers 101, 102 for which the movementmeasurements may be associated with external values (slope, turn from adata bank or data from a route marker system, etc.). In the case ofdivergence from this data, a risk of loss of adhesion of the vehicle maybe detected and, by extension, complement the information provided bythe system for detecting zero speed (locked wheel but vehicle inmotion).

The detector of loss of adhesion of the vehicle may also, if required,be coupled to at least one tachometer 108 of the axis of the vehicle inaddition to one of the first and second accelerometers 101, 102 so as tocompare their data for measuring the angular movement and respectivelythe longitudinal movement. By this means, the function of detecting zerospeed may be thus made even more secure.

Principal Abbreviations

X: longitudinal axis (of movement) of the vehicle

Y: axis perpendicular to the axis X and in the plane of the floor of thevehicle

Z: axis perpendicular to the floor of the vehicle

Px: plane at right angles to the axis X and determined by the axes Y, Z

Py: plane at right angles to the axis Y and determined by the axes X, Z

Pz: plane at right angles to the axis Z and determined by the axes X, Y

Gpes: acceleration of gravity=9.81 m/s2

Gx: longitudinal acceleration of the vehicle along the axis X

Glat: lateral acceleration of the vehicle at the location of theaccelerometers in the vehicle

Vx: longitudinal speed along the axis X

Dx: longitudinal position/movement along the axis X

VxT: longitudinal speed provided by the tachometer

DxT: longitudinal movement provided by the tachometer

Acc1: first measurement axis of the accelerometer 101

Acc2: second measurement axis of the accelerometer 101

Acc3: first measurement axis of the accelerometer 102

Acc4: second measurement axis 2 of the accelerometer 102

A1: angle in the plane Py between the axis X and the axis Acc1

A2: angle in the plane Py between the axis X and the axis Acc2

A3: angle in the plane Pz between the axis X and the axis Acc3

A4: angle in the plane Pz between the axis X and the axis Acc4

Ax: angle of trajectory of the vehicle in the plane Py (i.e. the anglebetween the horizontal and the axis X)

Lx: offset distance between the center of the vehicle and the fixingpoint of the accelerometers 101, 102

Ay: angle associated with the radius of curvature in the plane Py. Ay iscalculated by Arctg (Lx/R), thus in a first approximation Lx/R

Vx: longitudinal speed of the vehicle along the axis X

1-15. (canceled)
 16. A device for measuring a movement of a self-guidedvehicle, comprising, on-board the vehicle: a first accelerometer havingtwo first measurement axes in a longitudinal plane defined by a firstlongitudinal axis corresponding to a principal movement of the vehicle,assumed to be rectilinear, and of a second axis perpendicular to a floorof the vehicle; a second accelerometer having at least two secondmeasurement axes in a horizontal plane defined by the first axis and athird axis perpendicular to the first and to the second axis; a computerconnected to receive first output signals associated with each of thetwo first measurement axes, wherein each first output signal comprises aprojection measurement of a total acceleration resultant of the vehicleon the associated measurement axis, and connected to receive secondoutput signals associated with each second measurement axis, whereineach second output signal includes a projection measurement of a totalacceleration resultant of the vehicle on the associated measurementaxis; the first and second measurement axes of said first and secondaccelerometers having in their respective plane an adjustable relativeangle, such that, based on the four projection measurements, saidcomputer provides at least one value of longitudinal acceleration of thevehicle at each point of a route including slopes and turns.
 17. Thedevice according to claim 16, wherein at least one of the relativeangles is a right angle.
 18. The device according to claim 16, whereineach relative angle is subdivided into a first angle and a second anglecorresponding to projection angles between the first and secondmeasurement axes of the first and second accelerometer and the firstaxis.
 19. The device according to claim 18, wherein the projectionangles of each accelerometer are equal.
 20. The device according toclaim 16, wherein said computer is configured to provide at each pointof the route including slopes and turns a value of a lateralacceleration, a value of the slope angle, a lateral acceleration angleresulting from a centrifugal force due to a speed of the vehicle anddepending on a radius of curvature of a trajectory and an offset of theaccelerometer relative to the center of the vehicle.
 21. The deviceaccording to claim 16, wherein said computer is configured to provide aspeed and a position at each point of the route including slopes andturns by successively integrating the value of the longitudinalacceleration of the vehicle.
 22. The device according to claim 16, whichcomprises: a tachometer disposed on at least one axle of the vehicle andproviding tachymetric values of speed and position of the vehicle; acomparator connected to receive the tachymetric values and the speed andposition values provided by said computer; wherein said comparatordetermines differences between the categories of speed and positionvalues, and, if the values lie below a predefined threshold, a resettingof the speed and position values provided by said computer at each pointof the route including slopes and turns is implemented on thetachymetric values.
 23. The device according to claim 22, whichcomprises means coupled to said computer and to said tachometer fordetecting a zero speed of the vehicle, said means comprising at leastone correlator of the speed and position values provided by saidcomputer and the tachymetric values.
 24. The device according to claim20, which further comprises: an auto-calibration means of said first andsecond accelerometers to be activated if said zero speed detection meansconfirms a stoppage of the vehicle; said auto-calibration meansprocessing the measurements originating from said accelerometers andprovided by a unit for calculating accelerations incorporated in saidcomputer; and said auto-calibration means calibrating the measurementscorresponding to the zero values of the longitudinal acceleration andthe lateral acceleration of the vehicle.
 25. The device according toclaim 20, wherein said auto-calibration means has a first control modefor verifying a correspondence between the measurement values of saidsecond accelerometer and a means for recalculating the slope angle fromwhich the measurement values of said first accelerometer are verified ina second control mode.
 26. The device according to claim 24, wherein, onreaching a first error threshold arising from results of saidauto-calibration means, correction factors from the auto-calibrationmeans are transmitted to the computer.
 27. The device according to claim26, wherein, on reaching a second error threshold which is less safethan the first threshold arising from results from the auto-calibrationmeans, an indicator of failure of on-board measurement is activated. 28.The device according to claim 22, which further comprises means forevaluating a probability of failure that may be activated between twostops of the vehicle and using a redundancy measurement on themeasurement axes of the accelerometers.
 29. The device according toclaim 16, which further comprises a detector of loss of adhesion of thevehicle connected to at least one of said first and secondaccelerometers.
 30. The device according to claim 29, which comprises atleast one tachometer disposed at an axle of the vehicle and connected toreceive a signal from said detector of loss of adhesion of the vehicle.